The present invention is situated in the field of micro-electronics, more precisely in the field of the formation of porous silicon layers (PSL) and its lift-off to manufacture Silicon On Insulator (SOI) structures or to bond it to other low-cost substrates for the fabrication of photovoltaic cells.
When manufacturing a solar cell, only the top few microns of a silicon wafer will participate actively in the conversion of solar energy. Most of the expensive silicon wafer will only provide mechanical strength to the cell. This function can be achieved by any other low-cost substrate compatible to the solar cell fabrication process. Thus cost reduction is possible by providing a reduction in the use of silicon.
Silicon on insulator (SOI) structures are well known in the art, for example in the manufacture of low-cost solar cells with high efficiency. Some of the advantages are prevention of latch-up, low parasitic capacitance, high-speed operation and the absence of a need for a welling process.
In general, the existing techniques for the formation of PSLs and to transfer them to other substrates can be seen in FIGS. 1A through 1F and involve the following three steps:
Formation in a silicon substrate 1 of a low porosity layer 2 on the surface and a high porosity layer 3 thereunder by anodisation of silicon in hydrofluoric acid by changing the current density during the PSL formation.
Formation of a separation layer 4 under the high porosity layer 3 by high temperature annealing in hydrogen. This separation layer is a highly porous layer and is mechanically very weak. It can easily be broken by little mechanical force, e.g. by ultrasonic treatment or pulling.
Bonding of the obtained structure to another substrate 5 using an adhesive 6.
This process is disclosed in FIGS. 1A through 1F.
These techniques have been discussed by H. Tayanaka et al., 2nd world conference and exhibition on photovoltic solar energy conversion, Vienna, Austria, 1272 (1998), and by T. J. Rinker et al., Applied Physics A, 68, 705-707 (1999).
Another approach, disclosed by T. Yonehara et al. in Electrical Society Proceedings, Volume 99-3, 111, uses mechanical grinding, selctive etching and hydrogen annealing for transferring thin silicon epi layers on the other substrate. The European patent application EP 0867920 discussed the use of a laser beam for the separation of the thin layer from the silicon substrate.
All of the above-mentioned procedures for forming a thin porous layer and separating it from the substrate involve multiple steps, some of which are complex. Each step adds to the cost of the SOI structure or solar cell.
The primary aim of the present invention is to provide a novel, one step method for the production of porous layer films. A further aim of the invention is to provide a novel method for the production of porous layer films that allows good control over the pore depth. Another aim of the present invention is to provide a novel method for the production of porous layer films in which the substrate is available without aftertreatment after lift-off of the porous layer film for the production of a further porous layer film.
The present invention concerns a method for the manufacture of porous layers in a semiconductor substrate, comprising the following steps:
Providing a semiconductor substrate comprising at least one surface, said substrate serving as an cathode,
Providing a anode,
Applying a solution comprising Fxe2x88x92 ions, suitable for removing material from said substrate, between said surface and said anode,
Applying a predetermined current between said anode and said cathode, and
Maintaining said solution and said current a sufficient amount of time to obtain a low porosity layer at said surface and a high porosity layer positioned under said low porosity layer.
With this novel method, a porous semiconductor film comprising a low porosity and a high porosity layer can be obtained with a one-step method, i.e. without changing any parameter. More specifically, the solution will not be replaced and the current will not be changed. For the purpose of the invention, layer shall be understood as a part of the substrate that can be distinguished from another part of the substrate.
The amount of time the current and the composition of the solution have to be maintained depends on the value of the current and of the concentration of the Fxe2x88x92 ions in the solution and can be determined by the person skilled in the art based on the examples as disclosed in the detailed description. A Fxe2x88x92 ion containing solution can be obtained by dissolving HF, a buffer solution containing Fxe2x88x92 ions, NH4F, NaF or any other Fxe2x88x92 containing compound into an aqueous solution. Further, said solution preferably further comprises a wetting agent such as an organic solvent, preferably, but not limited to ethanol or acetic acid.
With the method of the invention, a porous layer comprising a low porosity layer and a high porosity layer positioned thereunder can thus be obtained.
The concentration of Fxe2x88x92 ions depends on the ionisation constant of the fluorine containing compound. Preferably, the concentration of Fxe2x88x92 in said solution is comprised between 10 and 40%.
Preferably, the value of said current is higher than 50 mA/cm2. The upper limit of the current is usually determined by the equipment and/or material constraints.
The method of the present invention can further comprise the step of halting the application of current for a short time, and reapplying the current to obtain a higher thickness of the porous layers. Halting the application of the current will allow the hydrogen to escape before its pressure allows horizontal cracks to occur. By repeating this several time, a porous layer with a desired thickness can easily be obtained.
In a further embodiment, the method of the present invention can further comprise a mechanical treatment to obtain lift-off of said low porosity layer and part of said high-porosity layer from said substrate. Said mechanical treatment can be any of the known treatments such as ultrasonic treatment, pulling, etc . . . .
In a second further embodiment, the method of the present invention further comprises maintaining said solution and said current a further amount of time in order to obtain lift-off of said low porosity layer and a part of said high-porosity layer from said substrate. This allows obtaining a separated porous layer film which can be attached to a suitable low-cost substrate. Said separated porous layer film can comprise said low porosity layer and a part of said high porosity layer. Said part of said high porosity layer can be understood as no high porosity layer or a certain thickness lower than the high porosity layer before separation.
In a third further embodiment, the method of the present invention further comprises applying a second aqueous solution, suitable for electro-polishing the substrate and for providing separation and lift-off of the low porosity layer and a part of the high-porosity layer from the substrate and for providing a new polished surface on the substrate, while maintaining the current. An alternative way to obtain the same effects is to apply in a further step a higher current, suitable for electro-polishing the substrate and for providing separation and lift-off of the low porosity layer and a part of the high-porosity layer from the substrate and for providing a new polished surface on the substrate, while maintaining the solution. By using this process, the semiconductor substrate surface is polished after removal of the porous layers and thus no aftertreatment of the semiconductor substrate is necessary to proceed to the manufacture of a further porous layer film.